The present invention relates to inkjet printing devices for printing secure images on media.
Inkjet printing systems frequently make use of an inkjet printhead mounted to a carriage which is moved back and forth across print media such as paper. As the printhead is moved across the print media, a control device selectively activates each of a plurality of drop generators within the printhead to eject or deposit ink droplets onto the print media to form images and text characters. An ink supply that is either carried with the printhead or remote from the printhead provides ink for replenishing the plurality of drop generators.
Individual drop generators are selectively activated by the use of an activation signal that is provided by the printing system to the printhead. In the case of thermal inkjet printing, each drop generator is activated by passing an electric current through a resistive element such as a resistor. In response to the electric current the resistor produces heat, that in turn, heats ink in a vaporization chamber adjacent the resistor. Once the ink reaches vaporization, a rapidly expanding vapor front forces ink within the vaporization chamber through an adjacent orifice or nozzle. Ink droplets ejected from the nozzles are deposited on print media to accomplish printing.
There is frequently a need to print documents that can be verified as original documents. Documents that can be verified as original documents are referred to herein as “secure” documents. Several examples of documents that require verification of their originality would be desirable include tickets, coupons, and various types of certificates, to name a few. For these printing applications it is necessary that the source of the document be verifiable by examination of the document. The technique used to identify the source of the document should be difficult to duplicate using readily available duplication systems such as copiers and scanners to prevent counterfeiting of the document.
There is an ever-present need for techniques for secure printing using inkjet printing technology. These techniques should be capable of allowing the source of the printed media to be identifiable without adding significant costs to the printing system. These techniques should be suitable for use with standard media. Finally, these techniques for authenticating original inkjet et printed documents should be reliable and easily accomplished.